Differential signaling is commonly used in high-speed digital signaling applications for a variety of reasons. For example, differential signals tend to require lower voltage swings than single-ended signals. This is true because the differential threshold in a differential receiver is more easily controlled than the threshold of a single transistor. Lower voltage swings lead to faster circuits and, quite often, lower power consumption.
Differential signaling also reduces electromagnetic interference (EMI), because the opposite currents carried on the coupled conductive traces of the differential signal line leads to cancellation, at large distances from the signal lines, of the electric and magnetic fields that occur in the signal lines. Similarly, differential signals are less sensitive to crosstalk.
Differential signals do, however, suffer from one significant drawback. When the layout of a pair of edge-coupled differential signal traces changes direction (i.e., makes a “turn”) on a printed circuit board (PCB), one of the two traces becomes longer than the other trace, and the perfect balance of the differential signal is degraded. This degradation in the differential signal, which is known as “delay skew” or simply “skew,” increases the EMI radiation from the differential signal pair and increases crosstalk.
FIGS. 1, 2 and 3 illustrate the phenomenon of delay skew in three types of turns taken by differential signal traces on PCBs. FIG. 1 shows the skew resulting from a square turn by the differential signal traces; FIG. 2 shows the skew resulting from an angled turn; and FIG. 3 shows the skew resulting from a rounded turn. In each example, it is easy to see that the distance traveled by the signal in one of the traces is longer than the path traveled by the other signal. In the example of FIG. 1, this increased distance, or skew, is equal to a+b, or 2 p. In the example of FIG. 2, the skew is equal to c+d+e+f, or 4(p·tanΘ), or 1.65 p. In the example of FIG. 3, the skew is equal to h−g, or (π/2)(r+p)−(π/2·r), or 1.57 p.
Designers of PCBs have traditionally tried to combat delay skew in differential signals by laying out differential signal traces so that every turn in one direction has a corresponding turn in the opposite direction. Such a technique can be very difficult to accomplish in practice, however, as the complexity of PCBs often does not allow the designer to match every turn in the differential signal traces with an opposite turn elsewhere on the board.